In general, steel cords used for reinforcement of carcass plies and belt plies in tires for construction vehicles and reinforcement of conveyer belts require high strength. Therefore, for these uses, a steel cord having a multi-twisted structure wherein strands having a plurality of steel filaments twisted together are further twisted together, is widely used.
On the other hand, to meet the demands related to environmental problems and improvement of energy efficiency in recent years, tires are demanded to achieve increase in the fuel efficiency and suppression of the cost of transportation by weight saving and reduction of the rolling resistance. Therefore, as a means to increase the strength of a steel cord, enhancement of the strength of filaments was attempted by changing the quality of their material (especially, carbon content) and/or the method of manufacturing thereof (e.g., reduction of area).
However, although a conventional method which enhances the cord strength by enhancement of the tensile strength of filaments was effective for a steel cord having a single-twisted structure or a normal layered twisted structure, it was not sufficiently effective for a steel cord having a multi-twisted structure comprising strands in which a plurality of steel filaments are twisted together. This is because, in such a cord having a multi-twisted structure, an increase in the filament strength is not directly linked to an increase in the cord strength, depending on the state of contact among the strands or the filaments. Thus, to solve this problem, various improvements have been carried out so far.
In terms of improvement of a steel cord having a multi-twisted structure, for example, Patent Document 1 discloses a technology for a steel cord for reinforcement of rubber, which steel cord has a 7×19 structure constituted by (1+6+12)+6×(1+6+12), to obtain a high cord strength by prescribing the tensile strength of filaments in the outermost layer of sheath strands and the tensile strength of filaments in the adjacent inner layer such that a specific ratio is attained between these. Further, Patent Document 2 discloses a technology for a steel cord for reinforcement of rubber articles, which steel cord has a multi-twisted structure, to obtain a high cord strength by setting the tensile strength of outermost layer sheath filaments constituting strands to not more than 3,040 N/mm2 and the tensile strength of all the inner filaments other than the outermost layer sheath filaments to not less than 3,140 N/mm2.
Further, Patent Document 3 discloses a technology to obtain a high cord strength by employing outermost layer filaments constituting strands thicker than the filaments in the inner side thereof to avoid preceding break of the outermost layer filaments. Patent Document 4 discloses a technology to obtain a high cord strength by prescribing the tensile strength of the filaments constituting each strand as “the filament tensile strength of the layer close to the center of the strand≧the filament tensile strength of the layer distant from the center of the strand” and the average tensile strength of the strands as “the average tensile strength of the strands in the layer close to the center of the cord<the average tensile strength of the strands in the layer distant from the center of the cord” to prevent preceding break of filaments.